This invention relates to a key telephone system, and more particularly a key telephone system wherein a standard telephone set is connected to a main control device through a pair of speech lines and is used as an office line or intercom with other key telephone sets. In the conventional key telephone systems, as the control transmission line for transmitting and receiving control signals becomes long, distributed capacitance results in attenuation of the data and delay of the data. Consequently, it it difficult to install a conventional key telephone set at a position remote from the main control device.
Efforts have been made to design a key telephone system comprising a combination of standard telephone sets which requires only a pair of speech lines and does not also require control lines.
To solve these problems, a system recently has been proposed in which a transfer switch is provided in an interface circuit. For example, when a standard telephone set is used for intercom speech, the standard telephone set is connected to a terminal in the main control device via one contact of the transfer switch and is connected directly to the office lines through the other contact of the transfer switch.
With such a connection, however, when an office line incoming call signal arrives it is necessary for the key telephone set to respond and transfer the signal. To transfer the signal, the user of a standard telephone must manually operate a transfer switch so as to impart a call signal to the main telephone set. When transferring the signal, intercom speech between a key telephone set and a standard telephone set becomes impossible. With a prior art key telephone system, therefore, manual operations were necessary for sending and receiving office line call signals.
In addition to a key telephone set used in a key telephone system, it is usual to accomodate a standard telephone set located at a relatively remote position which is used as a slave telephone set. The main control device is connected to the standard telephone set through a pair of connecting lines. A circuit including the connecting lines is usually connected to an intercom speech circuit by a selection circuit in the main control device. When a handset of the standard telephone set is off-hook, however, the slave telephone set could not be used because the intercom speech circuit would be monopolized.
To add other telephone sets to the key telephone system, additional substrates are used. In a telephone system of this type the substrates have been identified by a manually operated dip switch or by using an identification signal exclusively used for identifying the substrates. The dip switch not only increases the number of component parts but also may result in an erroneous setting. Where the identification signal is used to determine whether the substrate is used to add another key telephone set or a standard telephone set, it is necessary to install additional input and output lines for transmitting the identification signal.
When it is desired to install a key telephone set at a position remote from the main control device, attenuation of data results from line length and delay of the data results from distributed capacitance of the long line. Therefore, it is impossible to use a simple data transmission system. Instead, it is necessary to install a two wire standard telephone set utilizing a pair of speech lines and control circuits in and outside of the main control device, thus complicating the construction and increasing the cost of installation.
In a key telephone system wherein a call signal is sent to a telephone set through speech lines, a stored program system utilizing a contact of a loop supervisory relay has been used to stop sending out the call signal. A stored program system stores necessary processing procedures in a memory device according to a predetermined sequence. With this stored program system, howver, the stop of the call signal lags the hooking off of the handset by about 100 ms, including the operating time of the relay contact, a poling time and a timer operating time necessary for preventing erroneous operation due to chattering of the relay contact.